1. Field
The present invention relates to an illumination optical system, exposure apparatus, device manufacturing method, compensation filter, and exposure optical system.
2. Description of the Related Art
In a typical exposure apparatus of this kind, light emitted from a light source travels through a fly's eye lens as an optical integrator to form a secondary light source (in general, a certain light intensity distribution on an illumination pupil) as a substantial surface illuminant consisting of a large number of light sources. The light intensity distribution on the illumination pupil will be referred to hereinafter as “pupil intensity distribution.” The illumination pupil is defined as follows: by action of an optical system between the illumination pupil and a surface to be illuminated (a mask or wafer in the case of exposure apparatus), the surface to be illuminated becomes a Fourier transform surface of the illumination pupil.
Light from the secondary light source is condensed by a condenser lens and then superposedly illuminates the mask on which a predetermined pattern is formed. Light transmitted by the mask travels through a projection optical system to be focused on the wafer, whereby the mask pattern is projected (or transferred) onto the wafer. Since the pattern formed on the mask is a highly integrated one, a uniform illuminance distribution must be formed on the wafer in order to accurately transfer this micro pattern onto the wafer.
For accurately transferring the micro pattern of the mask onto the wafer, there is the proposed technology of forming the pupil intensity distribution, for example, of an annular shape or a multi-polar shape (dipolar, quadrupolar, or other shape) to improve the depth of focus and the resolving power of the projection optical system (cf. U.S. Pat. Published Application No. 2006/0055834).